1. Field of the Invention
This invention relates generally to pyroelectric sensors and, more particularly, to a method of determining the polarization state of a pyroelectric element by applying an AC signal to the element and calculating the hysteresis loop switching energy of the element where the spontaneous polarization reversal of the element is used as a time-varying function and the charge integration is directly proportional to the temperature of the element.
2. Discussion of the Related Art
A certain class of sensors make use of ferroelectric materials, and their pyroelectric effect for detection of temperature change. Sensors of this type have a wide range of applications, such as imaging in low visibility conditions, for example, poor weather conditions, night vision, etc. A ferroelectric material is a dielectric material that has a temperature dependent spontaneous electrical polarization in the absence of an externally applied electric field which can change state with the application of a critical field, where the polarization magnitude and direction within the ferroelectric material is identifiable by a hysteresis loop. The orientation of the polarization of the material can be changed by applying a reversing external electric field to the material. The electric dipoles within the material, that identify the orientation of the polarization, change when the external field is applied and in a proper circuit layout produce a hysteresis loop. Since spontaneous polarization is generally temperature dependent, ferroelectric materials can employ the pyroelectric effect for temperature detection purposes.
Any area of the hysteresis loop, either the entire saturated hysteresis area or merely a region of operation anywhere within the full loop, is representative of the switching energy required to change the polarization state of some or all the dipoles which make up the atomic lattice structure of the material at a given temperature for the specific state of excitation. Any change in radiation incident on the ferroelectric material, if absorbed, changes the temperature, and thus changes the associated loop area. FIG. 1 shows two charge versus voltage hysteresis loops for a particular ferroelectric material at a first temperature T1 and a second temperature T2. If plotted independent of physical dimensions, the magnitude of an externally applied alternating electric field is given on the horizontal axis and polarization, in charge density, is given on the vertical axis. The area of the charge versus voltage hysteresis loop of a ferroelectric material has dimensions of energy, and the loop area is a direct function of its temperature. The magnitude of the polarization changes with a change in the temperature of the ferroelectric material for a given electric field. A careful review of the two hysteresis loops in FIG. 1 will show that for the two different temperatures T1 and T2, the area within the loop is different. Consequently, an electrical measurement of the change in area anywhere within the major loop is an electrical signal corresponding to the change of the temperature of the material, and thus of the incident infrared radiation. The effect is of a dynamic nature due to the switching between polarization states of the pyroelectric material.
The spontaneous polarization Ps of a ferroelectric medium is a function of temperature T:
Ps=Ps(T)xe2x80x83xe2x80x83(1)
The pyroelectric coefficient p,
p=dPs/dT,xe2x80x83xe2x80x83(2)
is a temperature-rate of electric charge effect, that is often used as a pyroelectric quality factor for judging a particular ferroelectric material. The pyroelectric coefficient p is particularly sensitive in the vicinity of a Curie temperature, which marks a phase transition in the ferroelectric material. However, following a discreet change in temperature T, any external evidence of Ps, by its associated electric field Es, is only a transient phenomenon because of the unavoidable thermally generated free-charge, which rapidly neutralizes the Es. To overcome the transient nature of the measurable external evidence of Ps, all ferroelectric pyrometry to date has been based on the concept of forcing the temperature to become a function of time as:
T=T(t).xe2x80x83xe2x80x83(3)
Therefore,
Ps=Ps{T(t)}.xe2x80x83xe2x80x83(4)
Generally, the temperature T is converted to a time-varying function by mechanically shuttering a window between the heat energy source and the ferroelectric pyroelectric sensor. Unfortunately, the shuttering action rejects essentially one-half of the incident thermal power, which seriously decreases the signal-to-noise ratio. Furthermore, the need for a shutter between the sensor and the thermal source distinctly limits its use to the detection of only radiated heat, particularly infrared radiation.
A ferroelectric unit cell possesses two stable spontaneous polarization states Ps. This bi-modal condition exists while the selective material remains in a specific temperature range. The spontaneously separated xc2x1 bound-charge forms a dipole-moment. This dipole moment can be reversed by an opposing E-field if it is of sufficient magnitude. By locking in a residual orientation, the dipole memorizes the polarity of the most recently applied polarizing-reversing external E-field. This phenomenon is the operating principal of the ferroelectric random access memory (FRAM).
Coulomb""s Law suggests that the dipole-moment can be seen to represent stored potential energy, within the static, unexcited unit cell. At the instant of dipole reversal, and the immediate removal of the externally applied E-field, a spontaneous electric field Es would instantly appear across two hypothetical parallel plate electrodes positioned at the two surfaces of the cell that are oriented orthogonal to the dipole-moment. However, in a realizable, practical material at non-cryogenic temperatures, the omnipresent thermally-generated free charge will automatically migrate toward the bound charges, and reside in a posture so as to effect total neutralization of the externally observable Es field. The Es field must experience an exponential decay to zero, as determined by the resistivity times permittivity (xcfx81xcex5) time-constant of the material that can be expressed as:
Es=[Psxcex5]exp(xe2x88x92t/xcfx81xcex5).xe2x80x83xe2x80x83(6)
Therefore, it is clear that Ps in a ferroelectric capacitative structure, generally cannot be directly measured in a static manner, because Es as observed at the cell boundaries is a transient phenomenon. Consequently, to try to overcome the time-constant restriction, clever dynamic methods must be employed to effect a reliable measurement of Ps.
When a ferroelectric material, such as a crystal, ceramic, film, etc., consisting of numerous randomly oriented domains, each consisting of many such self-polarized unit cells, is excited with a time-dependent alternating electric field E, a time-independent display of Ps vs. E defines a directional hysteresis loop. A necessary condition for there to be any external evidence of Ps is that the period of the alternating excitation, 1/f must be short compared to the xcfx81xcex5 time constant as:
(1/ƒ) less than  less than xcfx81xcex5xe2x80x83xe2x80x83(7)
or
ƒ greater than  greater than (1/xcfx81xcex5),xe2x80x83xe2x80x83(8)
to insure that free-charge is denied the time necessary to neutralize the rapidly reversing bound-charge, and the Ps values remain essentially undiminished from their theoretical values. The area of the Ps vs. E loop has the units of energy density, i.e.,
xe2x80x83w=Total Energy W/Volume,xe2x80x83xe2x80x83(9)
or in other words, energy per unit volume.
Since Ps=Ps(T), the area of the time independent loop display of Ps vs. E is a direct measure of the temperature of the material. The electric displacement D in a ferroelectric material can be expressed as:
D=xcex5Exc2x1Ps=xcex50E+Pelasticxc2x1Ps,xe2x80x83xe2x80x83(10)
where xcex50 is the permittivity of free space.
In practice, the ferroelectric material must be structured with plate electrodes in the form of a capacitor. Therefore, the more practical measurement properties, namely, charge Q and voltage V, of a ferroelectric capacitor can then be written as
Q=CVxc2x1PsA=C0V+Qelasticxc2x1PsA,xe2x80x83xe2x80x83(11)
where A is the area of the electrode.
A portrayal of Ps vs. T will show that Ps diminishes abruptly as an increasing T approaches the Curie Temperature Tc, where Tc indicates a phase transition in the ferroelectric material. It is the abrupt change in spontaneous polarization over the narrow temperature range, albeit a transient phenomenon, that has been exploited in non-cryogenically cooled night vision systems, by introducing time-rate measurements.
The derivative of spontaneous polarization with respect to temperature is defined as the pyroelectric coefficient p:
p=dPs/dTxe2x80x83xe2x80x83(12)
However, the derivative of spontaneous polarization with respect to time is a pyroelectric current density given as:
Jp=dPs/dt,xe2x80x83xe2x80x83(13)
which is a measurable entity. Therefore, combining the two rate expressions,
Jp=p(dT/dt),xe2x80x83xe2x80x83(14)
provides a direct measurement of p, provided that there is a known and sufficient time-rate of temperature change.
In common practice, the time-rate of temperature change is accomplished by cyclical shuttering between the pyroelectric detector and the heat energy source, which is at the temperature that is to be measured. To overcome the foregoing time-constant degeneration of an externally measured Es, the pyroelectric sensor of area A is reset to a polarized state, the shutter is opened and the immediate current Ip is measured for a short time. However, in low energy level measurements, where the ambient noise equates to the signal energy, the shuttering technique is penalized by the requirement that approximately one-half of the heat energy is wasted by the shuttering operation.
To be complete, it must be understood that the temperature in all of the above discussion is the temperature of the ferroelectric material. If the sensor is intimately coupled to the object of which the temperature is to be measured, the accuracy is strictly determined by conventional thermal conductivity considerations. On the other hand, when the sensor is used to detect the heat radiated from a remote object, for example, infrared radiation, that energy must be received by an absorber, and then conducted to the sensor because the sensor itself does not customarily respond directly to infrared energy. The absorber is in effect an electromagnetic radiation impedance matching stub layer of intermediate impedance and specified thickness. Therefore, even though the ferroelectric sensor material is restricted to operation across only a narrow temperature range near the Curie temperature, the overall system can respond to a very broad range of radiation source temperatures.
Heretofore, all of the known ferroelectric/pyroelectric sensors that convert varying radiation energy to usable electrical signals greater than the inherent ambient noise of the sensor system operate in a passive mode. This means that the pyroelectric element operates at a given polarization state which is a function of temperature change, without any electrical polarization reversal. More specifically, passive pyroelectric detection only interrogates the polarization state of the ferroelectric material typically by measuring the net voltage across a poled capacitor structure, or by small signal AC excitation to determine the permittivity of the material (which is a function of the poled state), or some combination of these two methods. The practice in the industry to compare ferroelectric/pyroelectric sensors has been to measure the pyroelectric coefficient p. What this means is that for a physical geometry having sensor area A, the amount of coulombs of charge Q that are generated per degree Kelvin K, the pyroelectric coefficient p is expressed as: p=(1/A)[xcex94Q/xcex94K]. Unfortunately, this technique only represents a single cycle around a minor portion of the available signal energy as represented by the hysteresis loop area.
FIG. 2 shows a schematic block diagram of a known pyroelectric sensor system 10 that employs a conventional passive charge generation technique to determine the output of the sensor element. The sensor system 10 includes a chopper 12 that selectively gates radiation from a scene onto an infrared absorber 14 that is part of a pyroelectric element 16. The pyroelectric element 16 is made of a ferroelectric material that exhibits hysteresis loops which vary with temperature as shown in FIG. 1, and represents a single pixel element of the sensor system 10 that combines with other pixel elements (not shown) to generate an image, as is well understood in the art. The discussion herein is directed to an infrared imaging system, but as will be appreciated by those skilled in the art, sensor systems of this type are applicable to detect other wavelengths of radiation, including millimeter waves and microwaves.
The chopper 12 selectively blocks and passes the radiation directed to the pyroelectric element 16 at a predetermined frequency so that the pyroelectric element 16 sees a reference temperature when the chopper 12 is closed, and sees the temperature of the scene when the chopper 12 is open. The difference between the reference temperature and the scene temperature alters the shape of the hysteresis loop as shown in FIG. 1. The change in charge Q(t) 18 for the two loops is measured separately as a voltage across a sampling capacitor 20 and an amplifier 22, in a manner that is well understood in the art. Because no external electric field is applied to the pyroelectric element 16, the measured charge of the pyroelectric element 16 that charges the capacitor 20 for the two loops is the charge Q(t) where the hysteresis loop crosses the positive vertical axis for temperature T1 and the charge Q(t) where the hysteresis loop crosses the positive vertical axis for temperature T2. The sampling capacitor 20 stores the charge from the pyroelectric element 16 only each time the window is opened by the chopper 12. The effective pyroelectric coefficient p for this design is given as:
p=(1/A)[Q1-Q2]/[T1-T2]xe2x80x83xe2x80x83(15)
In an alternate known design, the small signal level capacitance, (i.e. change in local slope of the Q versus V curve of either a poled or unpoled ferroelectric material) of the pyroelectric element 16 is measured for temperature T1 and T2 and then compared. FIG. 3 shows a schematic: block diagram of a sensor system 26 including the chopper 12, the infrared absorber 14, the pyroelectric element 16 and the amplifier 22. Sometimes, a small bias voltage is applied to the pyroelectric element 16 from a bias source (not shown), and a capacitance meter 28 is used to measure the change in capacitance between the location on the hysteresis loop for both temperatures T1 and T2 relative to the bias voltage. Even though a small bias voltage is applied to the pyroelectric element 16 in this design, the mode of operation is still passive because the small bias voltage does not alter the polarization state of the ferroelectric material in any way, but merely measures its change in local permittivity as measured by a change in capacitance. The effective pyroelectric coefficient p is given as:
p=[(Vrms)/A](xcex94C/xcex94T)xe2x80x83xe2x80x83(16)
As is apparent, this detection scheme utilizes only a small portion of the hysteresis loop, and therefore the sensors are limited in their ability to differentiate signal from noise. Both of the techniques discussed above are dependent upon the condition that the ferroelectric material is left resident in one of its two spontaneous polarization states Ps (+ or xe2x88x92), or some intermediate stale thereof. The ability to measure the power from the pyroelectric element 16 between the temperature changes gives the sensitivity of the system. Because the signal-to-noise ratio is relatively low for the prior art sensors, this establishes the sensitivity of the entire system. Robust and relatively expensive system components, such as the chopper 12 and the amplifier 22 cannot increase the signal from noise, but only can prevent further degradation.
What is needed is a ferroelectric/pyroelectric sensor that measures more of the available signal energy from the hysteresis loop output from the pyroelectric element, and makes use of all of the heat energy available from the heat source to provide a better signal-to-noise ratio than is currently available in the prior art sensors. It is therefore an object of the present invention to provide such a sensor.
In accordance with the teachings of the present invention, a ferroelectric/pyroelectric sensor is disclosed that employs a technique of active excitation of the ferroelectric material by respectively changing its polarization state during a particular time period. An external AC signal is applied to the pyroelectric element to cause the hysteresis loop output from the element to cover a portion of the loop in accordance with the polarization direction change. Any suitable charge integration circuit can be employed to measure the charge from the pyroelectric element in response to the incident radiation. For example, a combination of a capacitor and operational amplifier can provide the charge integration, and a suitable rectifier and filtering circuit can be used to provide signal filtering. The sensor does not employ a chopper for providing a reference potential, but instead measures the charge from the ferroelectric material as a function of the spontaneous polarization of the material where the measured charge is directly proportional to the temperature of the material.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.